Oxidation of Lignin
The oxidation of lignin was performed in a 50 mL autoclave (Labe
Scientific Instrument Co., Ltd., Shanghai, China) with Teflon inner. In
a typical reaction, 0.1 g lignin, desired amount of catalyst and 10 g
pre-prepared microemulsion mixture were added in the autoclave, then
1.0 MPa O2 was used
to purge it three times and pressurized. Subsequently, the autoclave was
heated and kept at a designated temperature for 1-5 h under the stirring
rate of 600 rpm. After the reaction, the autoclave was cooled with
flowing water to room temperature. The reaction mixture was diluted by
ethanol to a fixed volume of 25 mL, among which 5 mL was fetched and
dried by anhydrous sodium sulfate for GC-MS analysis. Then, 50 mL
deionized water was added into other 20 mL solution for reprecipitating
lignin. So recovered lignin (Re-lignin) was obtained by the filtration
and then washed by water and dried for 4 h at 333 K for subsequent
characterization.
Measurement and
Characterization
The subregions of microemulsions were distinguished through electrical
conductivity measurements (DDSJ-308A, DSJ-0.1C electrode, Shanghai
Precision Scientific Instrument Co., Ltd).
During analysis, suitable amount of
acetic acid was added into the oil phase, with the concentration about
50 mmol L-1.27 The micro-polarities
of different microemulsions using methyl orange (MO) as the probe and
the solubility of lignin were measured by UV-vis spectroscopy (UV-2450,
Shimadzu, Japan) at room temperature with 0.1 nm resolution. Lignin
concentrations in different microemulsions were calculated by an
external standard method, where every test was carried out in
triplicate. Cryogenic scanning electron microscopy (Cryo-SEM) was
performed on a JSM-7100F SEM (JEOL Ltd, Tokyo, Japan) at 133 K. Before
analysis, the samples were pre-cooled by liquid nitrogen and stayed in
the preparation chamber (PP3010T Cryo-SEM Preparation System, Quorum
Technologies, UK) at 133 K. The formation of different microemulsions,
and the structures of raw lignins and Re-lignin were characterized by
FT-IR spectrophotometer (Bruker Tensor 27) by the KBr pellets with 4
cm-1 resolution and 64 scan times.
Volatile products were identified by GC-MS
(Agilent 7890B/5977A, HP-INNOWAX
capillary column: 30 m × 0.32 mm × 0.25 μm) on the base of Agilent MS
library. The temperatures of the injection and detector were fixed to
553 K. The oven temperature was started at 323 K (held for 1 min), then
programmed to 533 K at the rate of 10 K min-1 and
finally kept at 533 K for 15 min. The quantitative analysis was
performed by GC (Agilent 7890B) equipped with a FID detector using the
same chromatography column and temperature program as above GC-MS
analysis. The yields of phenolic monomers were measured by using
dimethyl phthalate as the internal standard and calculated through
following equations:
Y = \(\frac{A_{i}\text{\ M\ W}}{\text{A\ }M_{L}}\ \)x 1000 mg
g-1 (Equation 1)
Y = mg g-1 (Equation 2)
where Yi and Ai are the
yield and peak area of the product i , Y is the total yield
of phenolic monomers. M and ML represent
the mass of internal standard solution and lignin (g), W andA represent the mass concentration and peak area of internal
standard, respectively.